First things first; let's talk about the shirt:
1. The fit. Choose a t-shirt that actually fits your body. So many shirts are boxy and shapeless, and there's simply no amount of belting or accessorizing that will make an ill-fitting t-shirt look anything other than sloppy. Next, consider the cut of the jeans you'll be wearing the shirt with. If they're low rise, make sure you've got a t-shirt that's long enough to cover your middle section. The midriff is an area that should rarely be exposed at all, let alone at work. I like a shirt that gives me plenty of coverage without constantly having to tug at it.
2. The design. I'm not a big fan of writing on shirts, particularly at work. Because here's the thing: words written on t-shirts are typically emblazoned right across your boobs. And when people see writing on a shirt, of course they want to read it; it's just a natural human response. I'm sure I don't need to point out that in a work setting it's far better not to give anyone a reason to stare at your chest, whether the motivation is innocuous or not. Thus, the shirt should be harmonious with the rest of the outfit as a whole, and not be the only thing that draws attention. The easiest type of t-shirt to wear is one with a simple or abstract design and a soft enough color palette that the shirt doesn't distract from the rest of the outfit. Of course, there's also the possibility of wearing a shirt with a big statement-y image on it, but you'll definitely need to make sure you have some accessories and/or shoes that can really stand up to the design.
Now let's talk about the jeans:
1. The wash. The wash is really the main thing that sets a work-appropriate jean apart from a strictly weekend one (assuming it's ok to wear jeans at your workplace at all, of course). Even an ultra low rise pair of jeans can be fine for the workplace if you have the right amount of coverage from your shirt, but really nothing is going to make a lighter wash look anywhere near professional. Now my workplace is very casual and pretty much anything goes, so I can get away with wearing even my weekend pairs. But if you need a more professional look, choose a wash that's nearly black. Consider the difference in the outfits shown below, just in terms of the color of the denim:
Additionally, don't neglect the fact that the classic denim blue is very much a color and not a neutral. Any outfit with jeans should be designed with the color of the jeans in mind or else it'll look mismatched.
2. The cut and fit. Looking again at the 3 different outfits above, it's pretty easy to see that the wide-legged jeans have the dressiest and most formal feel. But I think that the real key to pulling off a stylish look with jeans is also about the fit. A good quality pair of jeans that flatters your figure is well worth the investment, in terms of not only money but also the time spent in searching for them. I'd much rather have 3 pairs of good quality, fantastically-flattering jeans than 10 pairs that are only so-so. I prefer jeans with a waistline below the belly button and just above my hip bone; for the most flattering fit, the waist of the jeans should sit right across that little tummy roll that most of us have.
Finally, let's discuss the rest of the outfit.
1. The accessories. In order to give a basic jeans-and-tee outfit some flair, it's really essential to add some interesting accessories. Today I topped my t-shirt with a fitted waistcoat and a double-wrap studded belt, and added my funky Betsey Johnson skull bracelet. With such a basic starting point as jeans and a tee, you almost can't overdo it with the accessories. Instead of the skinny studded belt and waistcoat I could have worn a big wide belt slung low on the hips, or a flashy scarf, or a hat, or several layers of long necklaces, or just about any combination of those.
2. The shoes. Ah, the pièce de résistance -- the shoes. As lovely Sal pointed out just the other day, a great pair of shoes can be the difference between a ho-hum outfit and a great one. They can be the defining factor that determines where the outfit is going to go. For this outfit, I was definitely seeking a little bit of punk rock attitude, so these plaid lace-up boots (which incidentally were a mere $26 on Amazon) fit the bill perfectly. The funky colors and pattern of the shoes took the emphasis off of the t-shirt, added some color, and made the whole outfit more interesting. The single biggest mistake I see people make with the jeans-tee combo is to throw on a boring pair of sneakers. Save the sneaks for when you're out jogging, and class up those jeans with some interesting kicks.
3/4 sleeve tee: H&M
Graphic tee: Gama Go